The postnatal development of the pineal organ of the ferret (Putorius furo) was investigated electron-microscopically with special interest given to the cerebrospinal fluid (CSF)-contacting pinealocytes and their large, vesiculated cilia.In the pineal of the newborn ferrets, there is a lumen —a pineal ventricle—which is a diverticle of the third ventricle of the diencephalon. The luminal surface of the pineal is bordered by ependymal cells and CSF-contacting pinealocytes. A sensory, 9×2+0 type cilium arises from the free surface of the pinealocytes and thickens in the first week. There are mitotic figures in the wall of the pineal ventricle, being reduced to a pineal recess during the second and third postnatal week.In two week-old animals, vesicles appear in the cilia of the pinealocytes. The vesicles may form rows and fill the enlarged cilium at the third week. Near the basal bodies, a proximal connecting piece remains narrow and free of vesicles. In older animals, there are multivesicular and dense bodies in the pineal cilia. The reduction of the pineal ventricle closes the CSF-contacting cilia in the intercellular spaces.Axon-like processes of pinealocytes form synaptic ribbon-containing terminals on secondary pineal neurons. Axons of pineal neurons enter the fiber bundles of the pineal tract running to the habenular nuclei. All these structures do not differ from the light conducting pathway of the submammalian pineals. The ultrastructure of the cilia investigated resembles that of the developing outer segments of the retina and represents a preserved light perceiving structure of the mammalian pinealocytes. Further studies are necessary to elucidate whether the early differentiation of the cilia and synapses indicates a timing of the circadian light rhythmicity in young ferrets by direct pineal photosensitivity.